Hydrogen bromide

Hydrogen bromide is the inorganic compound with the formula Template:Chem2. It is a hydrogen halide consisting of hydrogen and bromine. A colorless gas, it dissolves in water, forming hydrobromic acid, which is saturated at 68.85% HBr by weight at room temperature. Aqueous solutions that are 47.6% HBr by mass form a constant-boiling azeotrope mixture that boils at Template:Convert. Boiling less concentrated solutions releases H₂O until the constant-boiling mixture composition is reached.

Hydrogen bromide, and its aqueous solution, hydrobromic acid, are commonly used reagents in the preparation of bromide compounds.

File:HBr Dipole Moment.png

Molecular structure and dipole moment of hydrogen bromide (HBr)

Reactions

Organic chemistry

Hydrogen bromide and hydrobromic acid are important reagents in the production of organobromine compounds.<ref>Template:Ullmann</ref><ref name="greenwood">Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Butterworth-Heineman: Oxford, Great Britain; 1997; pp. 809–812.</ref><ref name="vollhart">Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and Function; 4th Ed.; W. H. Freeman and Company: New York, NY; 2003.</ref> In an electrophilic addition reaction, HBr adds to alkenes:

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The resulting alkyl bromides are useful alkylating agents, e.g., as precursors to fatty amine derivatives. Related free radical additions to allyl chloride and styrene give 1-bromo-3-chloropropane and phenylethylbromide, respectively.

Hydrogen bromide reacts with dichloromethane to give bromochloromethane and dibromomethane, sequentially:

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These metathesis reactions illustrate the consumption of the stronger acid (HBr) and release of the weaker acid (HCl).

Allyl bromide is prepared by treating allyl alcohol with HBr:

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HBr adds to alkynes to yield bromoalkenes. The stereochemistry of this type of addition is usually anti:

RC≡CH + HBr → RC(Br)=CH₂

Also, HBr adds epoxides and lactones, resulting in ring-opening.

With triphenylphosphine, HBr gives triphenylphosphonium bromide, a solid "source" of HBr.<ref name="hercouet">Hercouet, A.; LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157–158.</ref>

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Inorganic chemistry

Vanadium(III) bromide and molybdenum(IV) bromide were prepared by treatment of the higher chlorides with HBr. These reactions proceed via redox reactions:<ref>Template:Cite journal</ref>

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Industrial preparation

Hydrogen bromide (along with hydrobromic acid) is produced by combining hydrogen and bromine at temperatures between 200 and 400 °C. The reaction is typically catalyzed by platinum or asbestos.<ref name="greenwood"/><ref name="ruhoff"/>

Laboratory synthesis

HBr can be prepared by distillation of a solution of sodium bromide or potassium bromide with phosphoric acid or sulfuric acid:<ref name="brauer">M. Schmeisser "Chlorine, Bromine, Iodine" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 282.</ref>

KBr + H₂SO₄ → KHSO₄ + HBr

Concentrated sulfuric acid is less effective because it oxidizes HBr to bromine:

2 HBr + H₂SO₄ → Br₂ + SO₂ + 2 H₂O

The acid may be prepared by:

• reaction of bromine with water and sulfur:<ref name="brauer"/>

  2 Br₂ + S + 2 H₂O → 4 HBr + SO₂

• bromination of tetralin:<ref name="brauer"/>

  C₁₀H₁₂ + 4 Br₂ → C₁₀H₈Br₄ + 4 HBr

• reduction of bromine with phosphorous acid:<ref name="greenwood" />

  Br₂ + H₃PO₃ + H₂O → H₃PO₄ + 2 HBr

Anhydrous hydrogen bromide can also be produced on a small scale by thermolysis of triphenylphosphonium bromide in refluxing xylene.<ref name="hercouet"/>

Hydrogen bromide prepared by the above methods can be contaminated with Br₂, which can be removed by passing the gas through a solution of phenol at room temperature in tetrachloromethane or other suitable solvent (producing 2,4,6-tribromophenol and generating more HBr in the process) or through copper turnings or copper gauze at high temperature.<ref name="ruhoff">Ruhoff, J. R.; Burnett, R. E.; Reid, E. E. "Hydrogen Bromide (Anhydrous)" Organic Syntheses, Vol. 15, p. 35 (Coll. Vol. 2, p. 338).</ref>

Safety

HBr is highly corrosive and, if inhaled, can cause lung damage.<ref>Template:Cite web</ref>

References

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